The crystal structure of benzoic acid, C6H5CO2H, has been redetermined by X-ray difiraction at room temperature. Extensive neutrondiffraction measurements have also been made: by single-crystal methods at room temperature and 130 K; and, at 130 K and 5 K, by powder-profile analysis on C6D5CO2H.The structure consists of centrosymmetric dimers [formula(l)], in which two molecules are linked by a pair of hydrogen bonds between their carboxyl groups. Better precision attaches to the X-ray results. Full-matrix refmement, on 1011 independent reflexions, converged at Ä = 3.7 %. This refmement was indeed based on a model that was formally ordered, so far as concerns all atoms except the acidic hydrogen. However the structural results implied an averaged molecule, with the C-O distances 1.258, 1.268(2) Ä and the C-C-O angles 118.7, 117.8(1)°; and the acidic hydrogen appeared as two "half atoms" on the hydrogen bond, 0.9 Ä from each oxygen atom. These findings are most simply interpreted as due to disorder: the two configurations, A and B (of Fig. 1), occur randomly and in nearly equal proportions.Owing to difficulties inherent in the crystal texture of benzoic acid, the neutron results were less satisfactory. Large Single crystals were affected by twinning. Though the powder method avoids this difficulty, the structure, further confused by modulation, is rather too complicated for profile refmement. At 5 K however, the structure may be ordered, consisting wholly of dimers in the /1-configuration.

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The crystal structure of /3-DL-arabinose has been redetermined at 75 K, both by X-ray and by neutron diffraction. The cell dimensions and atom coordinates are in good agreement in both determinations, a = 5.8732 (8) [5.873(2)], b=7.7704(10) [7.762(2)], c= 13.2478(21) [13.243(9)] A,/3 = 99.96(I) [100.03(2)] °, V = 595.472 [594.55] .~3, Dr, = 1"674 [1 "677] Mg m -3, A = 0"7093 [0.8400] A (neutron diffraction values in square brackets). Electron deformation density maps around O atoms and the anomeric C atom have been computed by X-N methods. Experimental data concerning C-O bonds and O lone pairs are compared to theoretical ab initio studies on anomeric and exoanomeric effects. The most important points are: (i) lone pairs of O atoms 0(5) and O(1) seem to be quite identical except d2 of O(1) which interacts with the C(1)-O(5) bond; (ii) electron deformation density of anomeric C(5)-O(5) and O(1)-H bonds seems to be smaller than means in C-O and O-H bonds. These phenomena are consistent with predictions based on theoretical studies of anomeric and exoanomeric effects.

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